The first place where I was hired as a Dir. of MIS, considered Computer Word
Processing to be IBM Selectrics with memory!
"Howard C. Berkowitz" wrote:
> Excellent summary, Jenny. Let me add a couple of things. I'm just
> out of the hospital after getting traceroutes of my heart, and it
> seems vaguely appropriate to talk about SNA.
>
> SNA is an old technology, possibly dating back to the Garden of Eden.
> Is it any coincidence that the first three letters of evil are SNAke?
>
> Seriously, IBM did a great deal of very good work in making SNA
> reliable and extremely measurable. It isn't very flexible, but that
> wasn't a design goal.
>
> >Darren,
> > you are mostly right, but a bit mixed up.
> >
> >In SNA there are PU types and LU types.
>
> Another way to think of Physical Units (PU's, which don't have to be
> physical) and Logical Units (LU's, which seem illogical at times), is
> that PUs are network resources that do not directly involve
> application communications. While SNA doesn't precisely fit the OSI
> model, LU's are more at layer 6/7.
>
> PUs must activate before LU's can do anything. First, the SSCP (PU5)
> activates any FEP (PU4). PU4 functionality is built into the AS/400
> operating system and is not a separate box.
>
> Important: the Cisco IBM Interface emulates a 3172, which is a PU2,
> _not_ a PU4. You can get some, but not all, PU4 functions by adding
> IOS features. If you are considering replacing a FEP, ask yourself
> first if it could be replaced with an IBM 3172 before asking if it
> could be replaced with a CIP in a router.
>
> Anyway, in a mainframe environment, the SSCP, which owns a set of
> resources called a domain, activates the PU4's by creating SSCP-PU
> sessions. Each PU4, which is owned by the PU5, owns a set of
> resources called a subarea. IBM "logical addresses" are generally
> subarea/element. All PU and LU types can have IBM SNA addresses.
>
> Via the PU4, the SSCP then activates the various remote PU2s, with a
> couple of exceptions -- the SSCP directly activates the 3172 or CIP,
> not going through a PU4.
>
> Terminal controllers are PU2. Once they are activated, they can have
> one end of an LU on them. Think of each Secondary LU as a
> keyboard/display pair that can be assigned to a particular
> "application" under a primary LU.
>
> Primary LUs are more application service interfaces than true
> applications. An "application monitor" like CICS, IMS, or TSO
> actually provides services to applications, just as the OSI
> application layer is not itself an application but a set of functions
> that provides services to applications.
>
> >
> >The PU type describes the function/capabilities of the device within the SNA
> >hierarchy:
> >PU types:
> >5 VTAM (SSCP)
> >4 FEP
> >3 there is no PU type 3
>
> Correct. There probably was never a PU3. I have heard two
> explanations within IBM, one that there was an early proposal for
> lines to be PU3, and the other that two different SNA groups started
> numbering, respectively, from 5 and from 3. At least Rich McGee, who
> was head of the IBM SNA architecture group, and John Aschenbrenner,
> one of the principal designers, couldn't give me a better explanation.
>
> >2 3274/3174 cluster controller, 3777 RJE station, other devices (typically
> >connected to a FEP by a leased line running SDLC 1974-1980s) . Token ring SNA
> >gateway (1990s). SNA gateway on a LAN with a CISC router running
> >DLSw to another
> >Cisco router on a token ring (1990s-). Any SNA gateway product running on a
> >server will be a PU type 2.
> >1 I've forgotten what that was.
>
> PU type 1 were really stupid terminals. Reaching back in my memory
> archives, I think it was the 3176 that was an SNA-attached Selectric
> typewriter. Whatever the number, there was one. Frightening...a fair
> number of people on the list probably haven't ever seen a Selectric
> typewriter.
>
> >
> >
> >then when peer to peer networking came into SNA:
> >2.1 APPN (advanced program to program networking) supports independant LUs.
> >Includes more recent 3174 etc cluster controllers.
> >
> >
> >The type of the LU describes what data stream it uses to communicate. ie the
> >format of the data sent from one LU to another.
> >
> >LU types
> >new-fangled ones:
> >6.1 application to application CICS - not used much
> >6.2 application to application (APPC)
> >classic ones:
> >3 printer attached to a 3174 cluster controller using 3270 data stream
> >2 3270 display terminal (3278, 3179 etc) your basic "green screen", or
> >emulation of one on a PC
> >1 SNA character set - printer or RJE station
> >0 content of data stream not specified (this was used for program to program
> >communication before APPC/APPN came in eg Solve:Netmaster INMC, an Aussie
> >product)
> >
> >SLU :
> >in classical SNA a session is between two LUs. One of the LUs is the Primary
> >(PLU), the other the Secondary (SLU). The primary LU is the application on the
> >mainframe, such as CICS, TSO etc. The secondary LU is the user's terminal.
> >
> >Hope this isn't too overwhelming and it helps. Feel free to ask more
> >questions!
> >
> >Jenny
> >
> >---------------------------------------------------------------------
> >----------
> >Question for all:
> >
> >In the SNA hierarchy there are defined types 5,4,2 and 1 nodes.
> >
> >From what I've read so far I've assembled the following:
> >
> >Type 5 IBM Mainframe (Runs VTAM) SSCP
> >& PU5
> >Type 4 3705/3725/3745/3746 FEP/Communications Controllers PU4
> >Type 2 3174 Cluster Controller PU2
> >Type 2 Token Ring SNA Gateway PU2
> >Type 1 Token Ring Device LU
> >Type 1 TN3270 Terminals SLU
> >
> >Type 5 & 2 Cisco Router acting as DSPU Concentrator PU5
> >& PU2
> >
> >Is this anywhere near close?
> >
> >Does an ethernet client running a TN3270 session also appear as an SLU
> >or LU?
> >
> >Sorry if this is basic but SNA is the area that I am definately weakest
> >and while I can configure up basic DLSW+ services I prefer to understand
> >the underlying principles.
> >
> >Darren
>
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